The Trail of Hale-Bopp

March 31, 1997

Thousands of
images from professionals and amateurs are posted on the
Internet. This one was made on March 9 with a Pentax camera by a group
at Pearson College Observatory near Victoria, British Columbia,
Canada.

Comet Hale-Bopp's spectacular passage through the solar system has captured
the rapt attention of scientists and casual skygazers alike. From orbiting observatories
like the Hubble Space Telescope and ground-based telescopes, such as those at Palomar and the European Southern Observatory, to
amateurs equipped with telescopes, binoculars, opera glasses--or just the naked
eye--everyone is watching Hale-Bopp. Interest in this visitor from deep space
is peaking as the comet nears its closest point to the sun on April 1.

Hale-Bopp, which was first spotted by amateur astronomers Alan Hale and Thomas
Bopp on July 23, 1995, has more than lived up to its promise of being one of the century's "great comets." Clearly visible to the naked eye, it blazes so brightly that it
can be seen even before sunset and after dawn. At perihelion on April 1,
Hale-Bopp's double tail will be at its most vibrant as the sun's warmth
speeds the evaporation of the comet's frozen nucleus.

The show in the heavens is only part of the picture. Hale-Bopp has also
created a sensation on the Internet. There are a multitude of Web sites containing information on Hale-Bopp; they are attracting so many visitors that they are causing a traffic jam on the Internet. A Hale-Bopp homepage set up at NASA's Jet Propulsion Laboratory
was logging more than 1.2 million visitors a day over Easter weekend and had set up
two mirror sites.

Don't miss an opportunity to see Hale-Bopp with your own eyes--it won't pass
by again for another 4,000 or so years. But if it's a cloudy night, a bit of Web surfing
can be almost as good. Thousands of images are posted on various Web sites--some
almost in real-time, such as NASA's Comet
Watch. And, even if skies are clear, a tour of a few Web sites is well worth the
effort. You can check out the latest research findings, find out when and where
Hale-Bopp is visible in the sky, and get advice on how to make your own pictures.

Spinning in Space

The Internet also makes it possible to view this cosmic wanderer in ways not possible with the human eye. Because Hale-Bopp was unusually bright when it was
still a great distance away, well outside the orbit of Jupiter, it has given
scientists their best view ever of a comet. And they have recorded its progress
with a variety of instruments that capture changes in Hale-Bopp's nucleus in a
broad range of wavelengths, from infrared to ultraviolet. Other researchers have
created animations that
allow us to observe these changes over time. These data provide valuable clues to
the composition and structure of comets, which are believed to be remnants from
the formation of the solar system, about 4.6 billion years ago.

New research findings about Hale-Bopp are being released almost daily. In its
March 28 issue, the journal Science published a series of seven research papers about Hale-Bopp
observations. Included was a report on a year-long series of
observations by the Hubble Space Telescope of ultraviolet light emitted by
Hale-Bopp.

The Hubble team recorded a number of unprecedented events from this unusual
comet. The investigators observed the comet going through a
sudden brief outburst; in little more than an hour the amount of dust being
spewed from the nucleus increased at least eightfold. "The surface of
Hale-Bopp's nucleus must be an incredibly dynamic place, with 'vents' being
turned on and off as new patches of icy material are rotated into sunlight for
the first time," says Harold Weaver, a Johns Hopkins University astrophysicist
who headed the study.

The researchers also determined that Hale-Bopp is huge by comet standards. By
studying Hubble Space Telescope images, the astronomers estimate that its nucleus
may be 30 to 40 kilometers (about 19 to 25 miles) in diameter. The average comet is
thought to have a nucleus of about 5 kilometers (3 miles) in diameter. The comet
or asteroid that struck the earth 65 million years ago, possibly causing the
extinction of the dinosaurs, was probably about 10 to 15 kilometers (6 to 9 miles)
across.

Another group, consisting of astronomers from Cornell
University and NASA, reported results from tracking Hale-Bopp in the infrared
with a spectrometer and camera attached to the 200-inch telescope at Palomar
Observatory. The spectra show that Hale-Bopp has an abundance of tiny silicate
grains. Some of these grains are crystalline, in contrast to the more amorphous
structure of the rest. This means that the grains were subjected to strong
heating sometime in their history, before they were incorporated into the frozen
comet nucleus about 4.6 billion years ago.

"Our hope is that these dust grains, from under the surface of the comet's
nucleus, represent what the nucleus was like billions of years ago when it was
formed," says Thomas L. Hayward, Cornell senior research associate in astronomy
at the Center for Radiophysics and Space Research. "That could help tell us what
the solar system was like as it was forming."

As Hale-Bopp turns away from Earth on its lonely circuit through the
cosmos, observations will continue. For now, though, they are at a frenzied peak. The Hubble Space Telescope is temporarily out of action because pointing its instruments so close to the sun could damage them. But NASA's Polar spacecraft, whose primary mission is studying Earth's aurora, has turned to tracking Hale-Bopp during perihelion.

In addition, NASA is launching a series of four sounding rockets from White
Sands Missile Range, N.M., through April 5. The payloads, launched on two-stage
rockets, will observe the comet in the ultraviolet wavelengths of light for about
five minutes before returning to Earth by parachute. The resulting images will be posted to the
Internet by NASA. Also, the joint NASA/European Space Agency Ulysses spacecraft,
now in solar orbit, will study what happens to comets as they are exposed to
different solar wind conditions at various solar latitudes.

And over the next couple of years, ground-based instruments will remain pointed at
the receding Hale-Bopp. It will be years before all the information is
interpreted and made public. Then, Hale-Bopp will be gone, but not forgotten.